Falcao is on a season-long loan deal at United from Monaco which includes the option for the Barclays Premier League side to buy.
However, the Colombia international has scored only three goals so far in an injury-affected season and was left out of the squad for Sunday’s 1-0 home loss to Southampton.
Reports have suggested Falcao, who has been linked with Serie A side Juventus, was shocked by Louis van Gaal’s decision to leave him out against the Saints, a call the manager confirmed was tactical.
Van Gaal did also stress after Sunday’s game that the 28-year-old had started the previous five fixtures and that the next match would be “another moment” in terms of selection.
However, Mendes has raised doubts whether Falcao’s loan will be turned into a ??43.5m permanent transfer to United.
“He is a fantastic player, and I think it’s very difficult for Manchester United fans.
“They have an absolutely brilliant player, one of the very best in the world, who would without any doubt play 90 minutes of the match, every time, with any other club.
“The truth is, right now we don’t know what will happen.
“What we do know is that he will play in one of the very best clubs in the world next season, whether that is Manchester United or not.
“I am 100 per cent sure of that. But you know how football works, we need to wait a little. We’ll see what happens.”